I have got a SEPIC converter design see here:
http://www.linear.com/docs/27994
The calculation of the SEPIC converter starts at page 21.
For the input capacitance on page 17 they tell you need:
\$I_{RMS}(C_{IN}) = 0.3 \cdot ∆I_L \$
This is a current and not a capacitance how can I calculate a capacitance value from this current?
On page 24 they calculate the coupling capacitor current:
\$I_{RMS}(C_{DC})\$
but again this is no capacitance. How can I calculate a capacitance from this current?
Best Answer
From the datasheet (on page 17, right above your \$I_{RMS} = 0.3 \Delta I_L\$ equation):
This is basically saying: it depends. If your upstream source is close by and decently stiff, a smaller input capacitance (~10μF) will suffice. If the supply is further away or has a high output impedance, a larger capacitor (~100μF) is needed.
Similar to a boost converter, the input current is constant so the capacitors won't need to quickly supply a large amount of current. This is opposed to a buck converter whose input current is discontinuous and thus a higher amount of input capacitance is desired, especially for high-output-impedance upstream supplies.
In my experience, designing input capacitance can be kind of an iterative process. Put a reasonable amount (i.e. guess) in, observe the input voltage ripple (right at the input to the converter), and adjust as needed.